Spark gap electrical apparatus



May 14,1946. M. E HAINE ETAL: v 2,400,455.

' SPARK GA P ELECTRICAL APPARATUS Filed Dec. 18', 1943 i 2 Sheets-Sheet1 Negative I 0.0 Supp/y Oscillator Load Nejat/ve I L v WW y Oscil/azofiTr/p 4 a Lo d +c J F/GZ.

Negative I l p y T 3 Oscillator" I V. I (v/p Load +0 5 iucmuzr. mm 11mmJOHN IILLAR IEEK ATTORNEYS May 14,1946, M. E; HAINE; ETAL 2,400,455

SPARK GAP ELECTRICAL APPARA TUS Fil ed Dec. 18, 1943 2 S heats-Sheet 2"f /6,6 "1 aonnunmnmm ATTIORNL'YS Patented May 14, 1946 CsPAn-K GAPELECTRICAL APPARATUS Michael fEdwajrd "Haine and John Millar Meek, Sale,England, assignors to Metropolitan-Vicker's Electricalfflompany Limited,London, England, a compa'n'yo'f Great Britain Application Dece ber 18,1943, Serial No. 514,863

In Grea-t Britain July 11, 1941 '21 Claims. (01. 250 s7) This inventionrelates t o spark ga electrical apparatus adapted under predetermlnablecontrol to be tripped as nearly as possible at a predetermined instant,but more particularly at a plurality of accurately though notnecessarily equally spaced time intervals at recurrence irequencieswhich may be of the order of several thousand per second. H I I r Thepresent applicationis closely related to copending United Statesapplication Serial 7 No. 514,864, invented by Michael Edward Haine, and

copending United States application Serial No.

514,865, invented by Michael Edward Haine, John Millar Meekand JohnDrummond Craggs, both filed December 18, 1943. v b r I V v The primaryobject of the invention is to provide a trippable spark gap device which shall be reliable in operation as stated hereinafter, and theachievement or this presented a considerable problem which the featuresof the present invention as hereinafter pointed out in the appendedclaims have solved to an important extent as has been proved inpractice.

Part of the original secret problem was to employ atri ppable spark gapto provide heavy current substantially square-wave form or verysteep-fronted impulses of substantially constant amplitude of the orderof several hundred amperes each at recurrence frequencies of from onehundred to several thousand per second, the

impulses being sharply defined so as not to overlap one another andbeing in some cases of the order of one or a few micro-seconds durationeach. For the solution of this problem it was suggested to us'before thedate of the present- I are three spheres fixed in line, with the middleone biased so that the voltage is evenly divided across the two gaps inseries, the normal voltage between each gap being just too small tobreak down either gap. To cause the gap to break down an additionalvoltage or voltage pulse, whose amplitude is a few per cent of thenormal voltage existing acros the two gaps in series, is applied to themiddle sphere whereby one of the gaps becomes over-volted andbreak'sidown, resulting in that the full potential becomes applied tothe other gap which then also breaks down immediately afterwards, thissecond spark being the one of primary 'importance and being used for thevery high stressing of, for example, an insulator. Which of the two gapsbreaks down first, of course, depends on the relative polarity of thetripping voltage applied to the middle sphere. An important advantage ofemploying the middle sphere is'dueito the 'fact that it has a high andconstant circuit impedance 'to it's surroundings so that only a smallamount of energy 1 is'required to raise itspotential to the requiredvalue to initiate the main breakdown.

I The mechanism of breakdown of this threeball gap seemed to us the mostpromising with respect to the main object in view as above stated, afterconsideration of other known arrangements as follows.

Another method (see A. Kohler, Archiv. f. Elek. vol. 30, 1936, page 528)used to trip a spark gap which is stressed slightly below its criticalbreakdown stress, is to illuminate the cathode of the gap with the lightfrom another smaller spark in the vicinity, when the photo emission atthe cathode will cause the main gap to spark over. See inter alia W.Rogows'ki and Wallraff, Zeits. f, Physik. 9'7, 758, 1935; H. 'J. White,Phys. Rev. 1936, vol. 49., page 507; C. Brinkmann, Zeits. f. Physik.111, 737, 1939; and J. M. Meek, Proc. Phys. Soc. 52, 547, 822, 1940.

Disadvantages of these previously used methods for the predeterminedinitiation of the breakdown of a spark gap are the general uncertaintyin operation, the accurate adjustments required, and the care necessaryin keeping the electrodes free from dirt, etc, And, moreover, itisdifficult, though essential, to prevent the spark degenerating into anarc. A y

We are also aware of the particular spark gap devices disclosed inBritish Specification No. 267,929 (the application for the patent notbeing accepted) of which specification see Fig. 3, di-

' agrarnmatically illustrating. a lightning arrester arrangement for theprotection of a transmission line or other part of an electric powertransmission system, comprisingtwo metal spheres each having twoperforations all inline and each containing a pointed electrodeelectrically connected to the sphere through a choke reactance, the twopoints of these electrodes extending into the adjacent perforations ofthe spheres and facing each other: the one point electrode isv connectedto the transmission line or the like and the other is connected througha resistive impedance to earth.

We are further aware of British Specification No, 434,572 whichdescribes and claims a spark gap lightning arrester for the protectionof elec trical transmission and like systems, the arrangement being adifierent form of that described in Specification No. 267,929 just abovereferred to.

In connection with the protection of power transmission line systems, weare also aware of prior British Specifications Nos. 123,064 and 149,618,wherein are included spark gaps Of somewhat similar form to thoseconcerned in the present invention, but with important differences, aswill hereinafter appear.

With any protection system on a power transmission line system involvingthe spark gaps disclosed in the two prior specifications mentioned inthe preceding paragraph, and in two other prior specifications earlierherein mentioned, the arrangements are similar to that of the presentinvention, but only in so far as that the gaps must normally withstand ahigh voltage until triggered. However, the essential operativedifference between said previously disclosed spark gap systems and thatof the present invention arises after breakdown of the gap. In the caseof the present invention, after the breakdown of the gap the condenseror capacitative network is essentially such (and the appended claims asconcerned are to be interpreted accordingly) that the spark dischargeoccurs in one or a very few microseconds, and thereafter since theremaining or follow-up current is insufficient to maintain ionisationwithin the gap, the latter ceases to conduct and the gaseous ions arerecombined and cool rapidly until in a few milliseconds the gap canwithstand again the full voltage. In con tradistinction to this thetransmission line protector gap, at breakdown, passes an initial highsurge current and thereafter it passes a power line frequency currentsince in effect it essentially provides a low impedance path for thelatter to earth, and the effect of this current is the formation of ahot spot on the cathode electrode of the gap, and hence an arc is formedwhich must be extinguished or suppressed before the gap will cease toconduct, and this extinction or suppression is an impermissibly slowprocess with respect to the present invention. It can be achieved on theone hand by reducing the current to so small an amount that the arc canno longer be maintained: this has been achieved in protector gaps by theuse of material having non-linear voltage resistance characteristics,for example the substances known under the British Trade-Marks Thyriteand Metrosil. On the other hand, the extinguishing of the arc could beobtained by cooling it artificially such as by means of a blast of air,a method per se well known for circuit breakers, or by causing the arcto spread over a large area, also in per se well known manner, so thatit becomes stretched until it is unable to maintain itself. This isachieved in all the arrangements illustrated in the aforesaid priorBritish Specifications Nos. 123,064 and 149,618 by the provision ofarcing horns. Arcing horns are entirely superfluous with regard to theprimary object of the present invention qua operation. Since theyinherently provide a nonuniform field they could only be used with thespark gap of the present invention, namely only for the protection ofthe charging source against short circuit or overload, if arranged anddisposed so as not impermissibly to impair the cssential uniformity offield. In the arrangements diagrammatically illustrated in Figs. 6 and 8of British Specification No. 123,064 and in Fig. 5 of Specification No.149,618 it is possible that the presence of the arcing horns would notgreatly impair the uniform field between sphere gaps. It will be clearthat the arrangements of all the other figures of these two priorspecifications involve spark gaps which are arcing horns and are thususeless for the purposes of the present invention and are excluded fromits scope.

When, as a subsidiarily claimed feature of the present invention, an airblast is used, though it is for the purpose of cooling, the object isfor preventing, after a desired breakdown of the gap, the prematuresubsequent breakdown at too low avoltage.

We are of the opinion that the dimensions of at least the mainelectrodes of the aforesaid prior spark gaps for use for the protectionof power transmission lines are at least five, and probably ten, timesthe dimensions of any form of gap for any of the uses according to thepresent invention: see, for example, Electrotechnische Zeitschrift, July1934, pa e 736.

The dissimilar impedances essential in connection with the arrangementsshown in the aforesaid British Specifications Nos. 123,064 and 149,618are superfluous with respect to the claimed scope of the presentinvention.

Furthermore, in those of the appended claims which involve trippingspark gap electric systems or installations, power transmission linesare excluded, whilst in these claims a condenser or capacitative networkis to be interpreted as meaning a specially provided condenser or aspecially provided capacitative network (which latter may be what isknown as an artificial transmission line) for the essential purpose ofshaping the voltage or current wave, and the phrase does not mean atransmission line conveying power from a generating station orsub-station or the like, and the appended claims are to be inter pretedaccordingly.

Furthermore, since the present invention in operative conditionessentially requires that the discharge current in the gap must veryrapidly die away after each spark to avoid the latter degenerating intoan arc, there must be a high impedance between the power supply and thegap (as is provided by the charging impedance) to limit the currentafter the condenser or network has discharged.

Some of the spark gaps per se shown in the four prior Britishspecifications above mentioned, on paper, qua form, are somewhat similarto spark gap apparatus which we have evolved with the objects in view ashereinbefore and hereinafter stated, and one aspect of our invention maybe said to reside in the selection of such arrangements appropriatelymodified when used in particular circuits, but we are of the opinionthat these prior arrangements as disclosed and as would be used forprotection are unsuitable for the purposes in View, namely the veryaccurate and reliable control in point of time of the breakdown of aspark gap requiring in continued use no adjustment of the gap length andbeing independent, over a considerable life of the device, of otherconditions likely to be encountered in use. Our invention is notconcerned with lightning arresters for and when used in powertransmission line systems and such are herein specifically disclaimed.

According to another aspect of the present three-sphere gap upon theconsiderable-increase in the depression ofthe sparkovervoltage: thegreater this depresinvention, the spark gap thereof can be con- .namelyby reason of there normally being between these main electrodes asubstantially "uniform'field, whilst at least one trigger electrode isprovided which (or each of which) ises'sentially a slim member arrangedand provided in relation to the'main electrodes (which may be 'metalspheres or the like, including plates or part spheres) in such manner asnot normally to cause a field which will appreciably interfere with thenormal field between the main electrodes. Preferably the main-electrodeshave substantially'spherical ends fac'ing each other-{whilst one of themis formed 'with an axial Tperfior-ation towards or into or through whichextends the trigger electrode in'the form of a bluntly pointed wire.This trigger electrode, whilst being by reason of the external circuitsbiased to a i shorter gap between the trigger electrode and that mainelectrode with which it is associated. An important feature of theunitary structure spark gap device per se of the present inventionresides in the specific provision of means or material which effects,continuously or momentarily, irradiation of the gap adjacent the triggerelectrode such that the full double breakdown of the gap takes place,every time, with substantially no time lag, and with substantialcertainty. It is true that when the triggering voltage pulse is appliedto the trigger electrode, corona will be set up around the latter, butwe are of the opinion that, on the one hand, this occur too fortuitouslyfor regular controlled breakdown of the spark gap, so that, on the.

other hand, the addition of a separate and distinct source for ionisinggas at the electrodes is required. In the original apparatus which weconstructed the trigger electrode was mounted within the associated mainelectrode by means of solid insulating material. This arrangement gavesatisfactory results, and we have concluded that this was due to thestressing of, the gas by reason of the presence of this solid insulatingmaterial to produce the corona ionising the gas. It is per so commonknowledge that the initial ionisation required to cause the breakdown ofa spark gap maybe obtained by various specific means, more particularlyultra violet or X-radiation or radiation from radio-active material, andthe present invention in some of its aspects includes the specificprovision of these means or materials in substitution for or togetherwith solid insulating material or like corona-producing arrangements.The utilisation of cosmic rays solely is unsuitable for our purposes,though its uncontrollable effect cannot, of course, be excluded.

The success of the spark gap set "forth in the preceding paragraphdepends in part, we believe,

sion thegreater is the tolerance allowable on-the adjustment of the gapnecessary for accurate control of breakdown, and this is a feature ofmajor importance. Thus,- consider any spark gap which, undernormal'conditions and with no tripping voltage, wouldhave a breakdownvoltage of 100 kv. Now, supposethat with the aid of'the tripping deviceone can cause the gap to spark over at only kv., thus giVing a ten percent depression of sparkover voltage, then the application of thetripping'device will cause the gap to break down when it has any voltagebetween 90 *kv. and kv. upon it. Thus all other factors beingconstant,'one canwork over this range with no adjustment of'the .gap.How'ever,'in practice, due to change in atmospheric and barometricconditions, to dirt on the electrodes, etc.', thenormal breakdownpotential of the gap may vary by five or six .per cent, or even more, sothat for reiiable operation'onecan onl work between the limits 'of about90 kv. to 94 kv. This requires accurate adjustment of the gap initiallyand from time to time. We have arranged for the controlled trippingdevice to cause a depression of the sparkover voltage of thirty or fiftyper cent of the normal breakdown voltage of the gap. This is animportant advantage of the present invention when used for the trippingof an impulse generator apparatus where we have found a greatimprovement in reliability is obtained, also for spark-operated timesweep circuits for high speed cathode-rayoscillographs or in thesynchronous tripping of the impulse generator with the oscillograph timesweep and vice versa. The invention might also be used for thecontrolled triggering in otherwise per se known manner of spark gaps forcertain known photographic purposes. In the photography of objectsmoving at very great speed, such as projectiles, it is necessary to makeuse of very short exposures generallysuch that the object travels noappreciable distance during the exposure which may be of the order ofcnemicrosecond. By reason of this extremelybrief exposure time, itbecomes necessary that the illumination provided for efiecting theexposure should be correspondingly bright, and it is also necessary toprovide for controlling very precisely the mom'entof the occurrence ofthe flash. The foregoing photographic method has long been well knownand used. The spark gap and also the gap in association with thecharging circuits in accordance with the present invention mayadvantageously be used in connection with this method of photographysince the triggering can be very accuratel controlled, whilst the sparkprovides a very bright flash of short duration with a minimum ofafterglow.

-.'I'he most important use, however, is for the recurrent discharging ofa circuit. For such application an additional advantage is obtained byvirtue of the fact that, as the gap length is set appreciably or evenconsiderably in excess of the normal sparkover spacing, the tendency forthe spark to develop into an arc is avoided or reduced. In such acircuit we have, in our experi ments, run the gap at recurrentfrequencies of several thousands per second under conditions wherenormally it was extremely 'diific'ult t'o'prevent arcing through. v

The slim trigger electrode serves to provide when suddenly excited for avery brief period or a succession of periods a distortion of the normalelectric field in the main gap, and this distortion may be stated to beequivalent to the brief formation of a virtual point at the electrodewith which it is associated, resulting in a localised concentration ofthe field, and this will effect ionisation of gas leading cumulativelyto a discharge which rapidly spreads across the main gap, that isbetween the cathode or input electrode and the trigger electrode andthence to earth via the output electrode which is the one having thetrigger electrode associated with it.

The initial onset of ionisation requires, as hereinbefore stated, thespecific provision of ambient ions or electrons before the fielddistortion or concentration can cause the main spark breakdown of thegap. As is per se common knowledge stray ions necessary for the purposecan be produced by ultra violet radiation or X-radiation, and theinvention (in some of its aspects) involves these radiations (givingphoto ionisation) being pro; duced by soft X-rays or ultra violet raysemanating from the pre-breakdown corona discharge which is greatlyaugmented by reason of the in tense electric stress due to the presenceof a solid insulator adjacent the trigger electrode and the mainelectrode with which it is associated. It

will be appreciated per se that if the gap between a wire and asurrounding metal cylinder be partly filled b a solid insulatingmaterial between which and the wire there may be left a small gap, andif this insulating material has a dielectric constant greater thanunity, then when a high voltage is applied between the wire and cylinderthe voltage stress will be augmented in the gap between these metalmembers. Ionisation of the gas in this gap will thus occur at a lowervoltage than would be the case in the absence of the insulating materialand the suddenness of the application of the voltage ensures the coronabeing formed without time lag after the application of said voltage. Itwill be appreciated furthermore that adequate creepage distance must beallowed at the end of the solid insulator in order to prevent surfacesparkover taking place; thus in those forms of the invention employingsuch an insulator (as in the preferred forms), the insulator mustterminate a sufiicient distance from the ends of the main electrode andof the trigger electrode disposed therein.

The theory briefly indicated above on which we at present consider ourinvention is in part based, but to which we do not bind ourselves, willbe hereinafter further considered.

One of the important uses of the invention, as hereinbefore mentioned,requires that the spark gap shall on breakdown pass a relatively largedischarge current of about 50 amperes, with a low voltage drop acrossthe gap of the order of 200 volts, that is low in relation to theapplied gap I potential which may be of the order of 10 to 20 kv., eachdischarge being required to take place during a very brief period, forexample one or a few microseconds, and at very accurately spacedrecurrence intervals up to relatively great frequencies of, for example,3,000 or even 5,000 per second, the instant of breakdown being alwaysaccurately determined by the tripping voltage pulse. Considerationswhich then arise are, inter alia, (1) the reliability of operationthroughout a reasonable life of the device, that is in respect oferosion and corrosion of the electrodes, I and changes which may be madein the ambient gas, (2) avoidance of a tendency to are with relation tothe requisite recovery properties of the device after each maindischarge, this being a factor otherwise limiting the recurrencefrequency.

The main use of the invention, more specifically, is for the modulationof a generator or oscillator producing ultra high frequency oscillationsin pulses of very short duration and considerable peak power. Theresults achieved have been of successful importance. The generator oroscillator may thus be considered as a resistive load circuit into Whichthe energy stored in the charged condenser or network passes through thetrigger spark gap of the present invention and the electrical connectionof the oscillation generator is preferably such that it produces theoscillation pulses only during the instant or instants during which thespark gap breaks down.

The invention also includes further features which will be apparent onperusal of the appended claims.

In connection with the appended claims for triggerable spark gaps perse, we are aware of the following two prior British specifications:

No. 505,529, the arrangement shown in Fig. 5 of which has someresemblance to an enclosed spark gap according to the present invention,but there is the essential difference that in any arrangement accordingto thi specification, as will be clear to those skilled in the art, thecold cathode is specifically and deliberately constructed in such a waythat during operation at a few hundred volts maximum it will providethermionic heat as a result of glow discharge in the rarified gas orvapour at a pressure of a few centimetres at the most whereby to providein effect a thermionic emitting cathode. The construction of the cathodeis such that it will heat up vary rapidly. In any case, the result isthat the voltage drop in the gap is materially lowered; and there can beno spark, a spark being essential in any spark gap device of the presentinvention even although the gas pressure thereof may be somewhatsub-atmospheric. Any arrangement disclosed in this prior specificationis excluded from the scope of any relevant appended claim which is to beinterpreted accordingly.

No. 296,067 relates to apparatus for producing high frequencyalternating currents by means of spark gaps for therapeutic and likepurposes, and appropriate ones of the appended claims are to beinterpreted as excluding nitrogen alone at 400-700 mm. mercury pressureenclosed in a sealed chamber. The expression nitrogen alone is notintended to exclude the presence of small traces of impuritiesordinarily found in commercial nitrogen.

In the present specification and appended claims spherical or like asapplied to an electrode is to be understood as meaning an electrodeactive surface of large effective radius of curvature (includinginfinite) in relation to that of the slim or point-like electrode (aswill be well understood by those versed in the science of electricsparks) with respect to the operational purposes stated elsewhereherein.

In the accompanying drawings:

Figs. 1, 2 and 3 are diagrams of difierent circuit arrangements withspark gap devices and other features according to the invention.

Fig. 4 is a purely diagrammatic View of a spark gap per se in accordancewith the invention.

Fig. 5 is an elevation, half in section, of an enclosed and preferredform of the spark gap according to the invention, and

Fig. 6 is an underneath plan view of the device shown in Fig. 5.

Fig. 7 is a View of alternative means which may be used for reducingerosion of the tripping electheoscillator load 2.

trode of the spark gapaccordingto the invention: this alternative per seforms-the subject of copending United States application Serial No.514,864 aforesaid.

Figs. 1,2 and 3 will nowbe described;

The cathode Ia of the sparkgap device I has associated with it and thefeeding and current limiting choke coil L an artificial transmission.line in series with the gap anclload 2 a shown in Fig. 1 of'theaccompanying drawings, in which case the breakdown of the spark gapdevice I would cause a negative voltage pulse to appear across the valveoscillator, shown as the load 2, and it would be necessary to connectthe valve cathode to the anode 3 of the spark gap deviceand thevalveanode to earth. it is found in practice that difficulty isexperienced due to. the anode of the spark gap device and the-valvecathode rising to the high negative pulse potential. This difficultyarises mainly by reason of providing requisite insulation in connectionwith the means providing the recurrent tripping voltage pulse to thetrigger electrode 4.

spark gap device is earthed, as is one side of The artificial line 5 is,however, fully insulated from earth. It will be seen that the breakdownof the spark gap device I in this circuit will produce a voltage pulseof positive polarity across the oscillator load, so that the cathode ofthe said oscillator valve will be connected to earth. This is found tobe of advantage when a triode. valve oscillatorv as is per se known isrequired'to be modulated.

In the case of the oscillator being a magnetron, such as is being usedin connection with the invention, it is very desirable to earth theanode of the magnetron, and in this case it is expedient. to make use ofa phase reversing pulse transformer 2a as shown in Fig. 3. Apulse'transformer is one capable of transmittingshort pulses of voltagesfrom primary to secondary with no appreciable distortion of wave shape.

It should benoted thatthe use of the modified circuits of Figs. 2 and 3in'noway, affects the 4 t will be .well understood by those skilled theart that by a point electrode there isnot necessarily herein involvedone havinga very sharp point, the requirement being aIslim'triggerelectrode of such shape in relation tothe relatively large spherical orlike shape of the, main electrode forgiving the requisite main field, asto cause asuflicient distortion and concentration of this .field wherebythe depression of the. spark-over voltage .is of the order of thirty percent and upwards, accordingto varying conditions of use. With regard tothe life and stability of the device as, determined by electrode wear,we havefound that superior results are obtained. when the electrodes orparts thereof are formed of molybdenum or tungsten.

Referringnext to Fig. 4, the spark gap device according to the inventioncomprises a cathode Iztwhich is conveniently a sphere at least on thatpart thereof which faces the anode 3 which as shown comprises a tubularmember but having a substantially spherical left-hand end wherein thereis an aperture 3a through which extends the rounded, end of the slimtrigger electrode 4 which in the arrangement diagrammatically shown inFig. 4 is held within and spaced from the tubular anode 3 by theinsulating sleeve 4a which serves thexfurther purpose hereinbeforestated. The cathode Iahasa terminal or connector to, theanode-ihas'aterminal or connector 3: and the trigger 4 has its terminal or connector4. The insulator-4a may assume various other forms, conveniently thatshown at 41) inFig,/5 tobe hereinafter; described. Ashereinbeforeindicated the presence of the insulator 4a causesthepotential gradient to be augmented at'the surface of the triggerelectrode 4 for a given trigger voltage pulse so that the corona issuddenly produced along with the arrival of the tri ger voltage pulse.The radial width or the surface spark-over distance of the insulator "4must be large enough to obviate surface spark- .over on theapplicationof the trigger pulse. At

3" is shown a pipe going to the interior of the electrode 3, and at 3"is a pump for causing gas found that nitro-peroxide-was rapidly formed,v

causing the gap to are. We tried various gases of goodchemicalstabil-ity, particularly nitrogen and inert gas,especially.argon, at variouspressures-andwith varying results. It wasfound that in general argon individually was not very satisfactory athigh triggering pulse frequencies, due to the fact that the gas atomsattain a metastable state and recover therefrom slowly, that is to saythe atoms recover in a. time which may be as long as 0.001 or even 0.1second. A filling of nitrogengave poor results, probably. due to the.formation ofmetastable atoms a1so,' Whilst a filling of hydrogen gavegood operating results the life of the. device was shortened by reasonof too rapid wear of the electrodes, particularly the trigger electrode,possibly due to the forma- V tion-Yin the discharge of unstablehydridesof the electrode materials, which subsequently splitjup,

depositing the metal on the tube walls and on the electrodes in powderform.

We are awarethat in'Geiger counters the .presence of a quantity ofoxygen with the argon filling causes a rapid quenchingi; that is,recoveryfrom themetastable state: see Journal of The Franklin Institute,May and June 1941.

We have found by research that very good operational results areobtained when the gas filling comprises argon with from aboutone tofiveper cent oxygen content at pressures from 15 to pounds per squareinch. Even better results may be expected with such a' mixture atgreater pressures when it may be preferableto use metal containersinstead ofglass ones. This feature per se forms the subject of aforesaidUnited States copending-application Serial No. 514,865.

It will be appreciated that when our spark #gap device is enclosed inasealedchamber.itsoperasparking is verymaterially reduced.

Accordingtothe at present preferred ccnstructionalform of the inventionshown in Figs. 5 and 756,- the sparkgap I isenclosedwithintherelacomprising per se a pressed molybdenum sphericalportion fixed by the portion lb to the stem lc which passes through theseal id at the top of the bulb.

The molybdenum anode 3 is spot welded to metal straps 3b which arespot-welded to metal posts 8 one of which as shown passes through theseal 9 and is exteriorly connected to an insulated stranded conductor inwhich is soldered to a tab ll of an external contact member l2 fxed tothe tubular insulating cap 13'. The conductor I8 is continued from thetab I l, as shown at Ilia, to another contact member iZa spaced 90 fromthe contact member 12.

The trigger electrode is in the form of a rod 4 which passes through theseal 9 and is connected exteriorly to the insulated stranded conductor24 connected to the external contact member i which is symmetricallyspaced from each of the contact members l2 and I211. The insulation 419between the trigger electrode 4 and the anode 3 is in the form of aglass tube fused to the top of the seal 9.

With further reference to the theory of operation of spark gap devicesin accordance with the present invention, it may be explained that, asis per se known, the voltage gradient required to cause breakdown of thegap with uniform field is that which causes an electron avalanche, whichcrosses the gap to the anode and is of such intensity that the radialfield produced by the positive-ion space-charge in the avalanche systemis of the order of the interelectrode field since the space-charge formsan efiective point on the anode surface. A positive streamer thendevelops from anode to cathode to form a conducting filament bridgingthe gap.

The voltage required to cause the propagation of the positive streameracross the gapis much lower than that required to cause the electronavalanche which leads to the initiation of the streamer, and correspondsroughly to that for breakdown between a positive point and an earthedplane. The field distortion produced by the positive space-charge in theavalanche is simulated in the spark gap device of the invention by thefield distortion produced by the trip pulse on the trigger electrode,and the breakdown voltage of the gap is thereby reduced by an amountapproximately given by the difference in breakdown voltage for asphere-to-sphere gap and a gap of the same length between a positivepoint and an earthed sphere. Thus for a gap of '1 cm. between sphereseach of 1.3. cm. diameter, the breakdown voltage in air atnormalatmospheric pressure is 29 kv., whereas for the positive pointto-spheregap of the same length the breakdown voltage is 12 kv. approximately,which is a depression of sixty per cent.

We have found that the magnitude of the po-' tential pulse applied tothe point electrode also affects the depression of the breakdown voltageof the main gap. It not only creates a radial field at the point andinitiates a positive streamer, but it also augments the main fieldacross the gap. Thus the higher the applied pulse voltage the greater isthe depression, other conditions being unchanged.

Referring to Fig. 7 of the accompanying drawings, the arrangementtherein illustrated has for its object the reduction of the erosion ofthe tip of the slim trigger electrode 4. In preliminary explanation ofthis, it will be appreciated that tively thick walled glass bulb l, thecathode ia in'the case of spark discharges it is thecathodic electrodewhich suffers the more from erosion: in the caseof the'electrode Ia(Figs. 1-4) since this is of relatively large size the erosion is not ofimportance compared with that arising at the tip of the slim triggerelectrode 4 since it is the secondary spark from the latter to the anode3 which causes the erosion, this secondary spark carrying the wholecurrent .of'that of the main and all important spark between the cathodela and the electrode 4. In the arrangement shown in Fig. 7 there is onthe one hand connected between the anode 3 and earth a resistance 3c orinductance which is relatively low in value but is, say, ten timeshigher than theimpedance of the load 2 (Figs. 1-3) through which thecurrent of the sparks passes on tripping. On the other hand there isconnected between the tripping electrode 4 and earth the auxiliary sparkgap 40 which is adjusted to spark over at a voltage slightly in excessof the trip voltage. It will be appreciated that the operation of thearrangement shown in Fig. '7 depends on the relative impedances of thevarious electrodes. The electrod 4 normally has a very high impedanceto' earth, which may even be infinite: the impedances of the sparks arerelatively very low. On tripping a spark takes place between the cathodela and the tip of the tripping electrode 4, immediately followed by thesecondary spark from the latter to the edge of the perforation in theanode 3, so that approximately the voltage of cathode la appears acrossthe relatively low resistance 30 whereby the current in the spark fromthe electrode 4 to the anode 3 is limited while nearly the voltage ofthe cathode la remains on the electrodes 3 and 4. Immediately followingthe "secondary spark, therefore, is a spark across the gap 40, thecurrent in which tertiary spark is many times that of the secondaryspark; thus the erosion of the tip of the tripping electrode 4 may bereduced, for example, a thousandfold. It will be appreciated that thespark gap 4c'may be made robust.

In general for any given electrode spacing it is, of course, the uniformfield gap which requires the highest voltage for breakdown. The sparkgap arrangement according to the invention will 0perate withvoltagepulses of either polarity in conjunction with either polarity of thehigh volt- .age electrode of the main gap. However, the best 7 operationfor any given polarity of high voltage electrode is obtained when apulse of opposite polarity is applied to the fpoin electrode, and thegreatest depression of breakdown voltage occurs for a positive voltagepulse when the high voltage electrode is of negative polarity. The factthat the depression is not so great for a negative pulse as for apositive pulse may be explained, since the'negative streamer does notpropagate as readily as a positive streamer, and the voltagev requiredtocause breakdown between a negative point and an earthed plane is abouttwice that for a positive point, although it is still considerably belowthatfor the uniform field.

It will be appreciatedthat in the arrangements of spark gap illustratedin the drawings with the active end of the trigger electrode 4approximately coincident with the active surface of the main electrode3, the potential at which the electrode 4 is normally biased is thesame, or

nearly the same, as the normal potential of the prising in combination,a circuit having'capacitytherein and means for connecting it to a sourceof current, an impedance connected in said-circuit for charging saidcapacity to a high potential, a load connected across said circuitthrough which the charged capacity is dischargeable at predet'erminableinstants of time, a spark gapdevice with which said load is connected inseries, said for applying a normal potential bias to said trig gerelectrode, said trigger electrode being so formed ahdlocated that whennormally appropriately biased there is so little disturbance of thesubstantially uniform field between said main electrodes that breakdownbetween them "is avoided with appreciable tolerance, and when atriggering pulse of sufficient magnitude is applied to said triggerelectrode breakdown is caused due to the appreciable distortion of saidfield.

2. A tripping spark gap electric system accord-' ing to claim 1, whereinat least one of said trigger electrodes is located sufiiciently near toone of said main electrodes that on the application of a triggeringpulse to said trigger electrode a main spark occurs between the other ofsaid electrodes and said trigger electrode followed immediately byanother spark in series between said trigger electrode and said mainelectrode with which it is associated.

3. A tripping spark gap electric system according to claim 1, wherein aportion of said trigger electrode has a relatively small radius ofcurvature which is directed towards one of said main electrodes.

4. A tripping spark gap electric system according to claim 1, whereinone of said main electrodes has a perforation, and said triggerelectrode extends from behind towards said main electrode and towardsthe other of said main electrodes whereby a radial gap is providedbetween said trigger electrode and the edge of said perforation.

5. A tripping spark gap electric system according to claim 1, whereinsaid main electrode with which said trigger electrode is associated hasa circular perforation, and said trigger electrode is of smallcross-section and has a rounded active end portion.

6. A tripping spark gap electric system according to claim 1, includingadditional means for irradiating the space between at least two of saidelectrodes.

7. A tripping spark gap electric system according to claim 1, includingmeans for providing ionizing radiation in the space between at least twoof said electrodes by the application of said triggering voltage pulse.

8. A tripping spark gap electric system accord-.

ing to claim 1, including means to augment the gradient at the surfaceof said trigger electrode for a given trigger voltage pulse, comprisingsolid insulating material located between said trigger gering voltagepulse a corona discharge which emits ionizing radiations to causesparkover.

10-. A tripping spark gap electric system according to claim ,1,including means for'causing a fiow.

of a gaseous medium between said electrodes for thepurpose ofpreventing, after a desired breakdown between them, a prematuresubsequent breakdownat too low a voltage.

11. A tripping spark gap electric system accord ing to claim 1, whereinone of said main electrodes has a perforation toward whichsaid triggerelectrode extends, and'is tubularand provided with means for admitting agaseous medium to its in- I terion to flow around said tri'ger'electrode and through said perforation; I

12. A-tripping-spark gap electric system accord-' ing to claim 1,including a sealed chamber enclosing said electrodes and containing aninert gas mixed with a small proportion of a suitable quenching gas.

13. A tripping spark gap electric system according to claim 1, includinga sealed chamber enclosing said electrodes and filled with argon.

14. A tripping spark gap electric system according to'claim 1, includinga sealed chamber enclosing said electrodes and filled with an inert gasmixed with oxygen in the range of proportions of from one to fivepercent, of oxygen.

15. A trippable spark gap device, comprising as a unitary structure,input and output main electrodes having spherical surfaces and fixedapart appreciably beyond their normal sparkover spacing in relation to apredetermined voltage to be normally applied across the gap, and atleast one fixed slim trigger electrode associated with one of said mainelectrodes and so formed and located that when normallyappropriatelybiased there is so little disturbance of the substantially uniform fieldbetween said main electrodes that when the normal voltage is appliedbreakdown is avoided with appreciable tolerance, and when a triggeringvoltagepulse is applied breakdown is caused due to the appreciabledistortion of said field, and a gaseous medium above atmosphericpressure enveloping said electrodes.

16. A trippable spark gap device, comprising as a unitary structure,input and output main electrodes having spherical surfaces fixed apartappreciably beyond their normal sparkover spacing in relation to apredetermined voltage to be normally applied across the gap, and atleast one fixed trigger electrode associated with one of said mainelectrodes and so formed and located that when normally appropriatelybiased there is so,

little disturbance of the substantially uniform field between said mainelectrodes that when the normal voltage is applied breakdown is avoidedwith appreciable tolerance, and when a trigger?- ing voltage pulse isapplied breakdown is caused due-to the considerable distortion of saidfield, a gaseous medium above atmospheric pressure enveloping saidelectrodes, and mean for causing the gas in at least one of the sparkgaps to be irradiated by ionizing radiation.

17. A trippable spark gap device according to claim 16, wherein saidmeans for irradiating said spark gap by ionizing radiation comprisessolid insulating material located between said trigger electrode andsaid main electrode with which it is associated' 18. A trippable sparkgap device, comprising as a unitary structure, input and output mainelectrodes having spherical surfaces fixed apart appreciably beyondtheir normal sparkover spacing in relation to a predetermined voltage tobe normally applied across the gap, and at least one fixed slim triggerelectrode so formed and located that when normally appropriately biasedthere is so little disturbance of the substantially uniform fieldbetween said main electrodes that when the normal voltage is appliedbreakdown is avoided with appreciable tolerance, and when a triggeringvoltage pulse is applied breakdown is caused due to the considerabledistortion of said field, and a gastight chamber enclosing saidelectrodes and containing a gaseous medium.

19. A spark gap device according to claim 18, wherein said slim triggerelectrode is associated with one of said main electrodes and is directedtowards the other of said main electrodes.

20. A spark gap device according to'claim 18, wherein one of said mainelectrodes has a perforation, and said trigger electrode comprises a rodwhich extends from behind said main electrode and into said perforationand towardsthe other of said main electrodes, so that a radial gap isprovided between said trigger electrode and said perforated mainelectrode.

21. A trippable spark gap device comprising, as a unitary structure,input and output main electrodes having spherical surfaces fixed apartappreciably beyond their normal sparkover spacing in relation to apredetermined voltage to be normally applied across the gap, and atleast one fixed slim trigger electrode associated with one of said mainelectrodes and so formed and located that when normally appropriatelybiased there is so little disturbance of the substantially uniform fieldbetween said main electrodes that when the normal voltage is appliedbreakdown is avoided with considerable tolerance, and when atriggeringvoltage pulse is applied breakdown is caused due to the appreciabledistortion of said field, said electrodes being exposed to theatmosphere, and a terminal member for each of said electrodes for 20connecting it to an appropriate circuit.

MICHAEL EDWARD HAINE. JOHN MILLAR MEEK.

